In general, the moisture in a food is an important factor affecting its physicochemical properties, storage life, texture and flavor. For example, physical deteriorations in shape and hardness due to the evaporation or absorption of water, microbial deteriorations due to fungi and bacteria and chemical deteriorations such as color change, fading out and oxidation of fats are serious problems in the preservation of foods. In order to cope with these problems, it is essential to control the moisture content and water activity of foods.
Namely, a food contained in a closed space has a certain vapor pressure. The ratio of this vapor pressure to the saturated vapor pressure is referred to as the water activity. In order to prevent the above-mentioned deteriorations, there have been known methods for effectively storing foods by using desiccants to thereby lower the water activity or maintain it at a constant level. A number of desiccants have been applied to foods. These desiccants are generally used by introducing into a closed container separately from a food. On the other hand, it has been required to develop edible desiccants which can be directly contacted with or added to foods and there have been proposed some materials for producing such desiccants. For example, silica gel can be directly added to foods, though the acceptable content and taste thereof restrict its addition.
On the other hand, anhydrous maltose described in JP-A-62-125854 and anhydrous raffinose described in JP-A-62-152537 are usable as edible desiccants (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However these substances are still unsatisfactory in properties relating to water activity. It is, therefore, urgently required to develop an edible desiccant of improved characteristics. Furthermore, anhydrous raffinose becomes deliquescent when absorbing water, which makes its food processing properties poor.
Regarding nystose, there have been reported crystalline nystose trihydrate obtained by recrystallizing from water/methanol [m.p.: 129.degree.-131.degree. C.; H., Tsuchida et al., Agric. Biol. Chem., 30, 429-433 (1966)], crystalline nystose monohydrate obtained by recrystallizing from ethanol/methanol [m.p.: &lt;134.degree. C.; J. P. Kamerling et al., Carbohydr. Res., 25, 293-297 (1972)] and amorphous nystose anhydride obtained by heating the crystalline nystose trihydrate at a high temperature under reduced pressure [no m.p. is given; H. Tsuchida et al., Agric. Biol. Chem., 30, 429-433 (1966)]. However it is reported that none except the amorphous nystose anhydride shows water absorption characteristics.